Sound reproducer or recorder head



Feb. 28, 1950 M. L. GAYFORD SOUND REPRODUCER OR RECORDER HEAD Filed Jan. 5, 1946 Patented Feb. 28, 1950 Michael. Lawrence Gayford, London, lilngdand,"

assi'gnor, by mesne assignments) tosln'ternai tional Standard- Electric -.Corporation,.-. New York, N. Y., a corporation of Delaware Application anqaiija me, semis time" In Great Brit in January 26' 19A 2 claims. (in. lie- 100.41)

invention relates to electricsound re-.

cording and reproducing heads, and more par-,

particularly to the construction and mounting of the armatures therefor. The invention is described with reference to devices of the balanced armature type, wherein an armature is located betweentwopairs of juxtaposed pole-faces and;

is turned about an intermediate axis with a,push-. pull effect which is known to eliminate or at least reduce second-harmonic distortion (see e. g. Olson, Dynamical Analogies, page 135).

is found to reside, and Figs. 3 and, 4 show two forms of this element in greater detail.

In both forms the armature itselfconsists of a single substantially rectangular block l5 of highly.-magnetisable-material. This block |5 is formed with integral-lugs lfi projecting in line from-the opposite side-edges andslotted ver-' tically so' that each lug can-receive a'spring strip It is known too that in soundrecording and reproducingheads it is highly desirablev to reduce the moment of inertia of the armature as much as possible, and this especiallywhere a wide frequency band is to be dealt with. Oneof the objects of the present invention is to attjainlj this desidefratum of reduced moment of inertia.

Theinvention is an improvement in or modificatiion' of 'theinventi'o'h disclosed in the copending U. S patent application Ser.. 1\,lo.619,652, filed October 1', 1945,'now abandoned."

The main featur'es'ofthe'invention are set out Fig. '1' is a front view of 'an electrical recorder head in accordance with the present invention, certain parts being removed for the sake of clarity.

Fig. 2 is a sectional view on the line AA of Fig. 1, and

Figs. 3 and 4 are side views, on an enlarged scale, of two forms of armature assembly utilised in the recorder head shown in Figs. 1 and 2.

An electrical recorder in accordance with the present invention is shown in Figs. 1 and 2. A U-shaped permanent magnet I has pole-pieces 2 and 3 held magnetically against its bottom extremities and extending inwardly towards one another. These pole-pieces 2 and 3 are bifurcated to form upper limbs '4 and 5 and lower limbs 6 and l. Plates 8 and 9 of brass cover the front and rear sides of the pole-pieces 2 and 3, extending across the ends of the gap between them and being secured by screws l0 passing into tapped holes I in the pole-pieces 2 and 3. In Figure 1 the front plate 8 has been omitted for clarity. Within the gap between the pole-pieces s arranged an armature |2 carrying a stylus I3.

coll I4 is located in the horizontal space beween the upper and lower limbs 4, 5, 6, I of the ole-pieces 2 and 3 so as to surround the armaure I2. It is in the construction and mounting f this armature l2 that the present invention I1 ofnon-magnetic material which is riveted or spot-welded in position as indicated at l8.

Being assembled; the armatureis held'in position:intheinstrumentby aclamping of the extremities of -the springstrips ll.

plates '8 and 9 have-inturned upper andlower portions-.19.-and:20-and-these are slotted to receive;.,the.extremities of the spring strips which are therelsoldered. Thus the armature plate I5 being-fixed ,byits lugs -|:6 to the spring strips |1 whose-extremities are clamped in a fixed part,

is mounted-for vibration about an axis through the lugs |6.-

Referring; again to Figs. 3 and 4, it will be seen that v ,the -.-lugs ;.||5 and hence the vibrating axis' are located in the=plane of the substantially'recj tangular plate l5-asomewhatbelow the centre.- The exact location is so chosen that the moment of inertia of the vibrating armature assembly; shall be a minimum. In pursuance of this same object of reducing the moment of inertia, both the stylus socket and a clamping screw 2| for the stylus, are recessed directly into the block. l5 itself.

Figs. 3 and 4 difier from one another in the arrangement by which the stylus I3 is clamped in position. In Fig. 3 the clamping screw 2| slopes towards the shank of the stylus and makes direct contact with it. In Fig. 4 on the other hand the screw 2| lies parallel with the stylus. This screw in Fig. 4 has a tapered head which engages in an oval tapered recess in the armature. The long axis of the oval is at right angles to the plane of the armature. Two slots 22 and 23 ending in two holes 24 and 25 are cut right through the armature to form a tongue 26 which is forced over by the screwing in of the taper-headedg screw 2| to grip the stylus shank at the bottom, Here a stylus with a longer shank is used and it goes into a hole which extends vertically through the entire height of the armatur The actual clamping point is at the bottom ed of the armature and hence as near to the actual cutting point as possible.

The hole in the armature is preferably given, a slight reduction in diameter towards its top Thus, referring backctot-Fig. ;2 ,--it Will-be seenthat the brass end and the stylus shank given a similar taper. The stylus is pushed up into the hole as far as it will go and is thus held tightly at its to end. It is then clamped at the bottom end by screwing in the adjacent screw in the armature.

Thus we have the stylus shank held tightly at both the top and bottom of the armature plate, hence avoiding any flost motion or rocking of the stylus shank about the point of clamping. The stylus is also gripped so as to give a minimum distance, and hence minimum flexibility, between the lower clamping point and the cutting point. This is desirable, as one of the factors fixing the upper cut-off frequency. of then recorded frequency response is the inevitable slight flexibility of the stylus between its clamping point and the cutting point. In this case the distance between these two points is reduced.

to a minimum and is considerably less than in Fig. 3, where the clamping screw, running in at an angle, gripped the stylus at about half-way up the armature.

As indicated at 22 in Fig. 3 the thickness of the block 15 is reduced in those parts remote from the stylus and screw sockets. With this variation in thickness of the block it may be necessary to vary the width of the air gap as between the upper pole-piece limbs 4 and 5 and the lower limbs 6 and 'I: thus it will be seen in Fig. 1 that the limbs 4 and 5 are slightly longer than the limbs 6 and I. In Fig. 4 owing to the use of a longer stylus shank (present standard commercial stylus) the armature is not cut down at the top. This could be done if the stylus shank and tapered screw were reduced in length. In a reproducing head, if the stylus were of sapphire, which will last indefinitely, it would be permanently in position, so that there would be no need for the provision of clamping screw 2|.

The pole-pieces and armatures of the instrument are of an alloy, of high magnetic permeability which will provide the maximum flux combined with low alternating reluctance and low eddy and hysteresis losses. The permanent magnet is of an alloy, in which the magnetic properties have been enhanced by heat. treatment in a magnetic field. The spring strips 11 are of a material, such as beryllium-copper,

which combines strength with resistance to mechanical fatigue, and which is non-magnetic.

It may be noted that whereas in earlier designs, such as that of the recording head shown in the copending U. S. patent application Ser. No. 619,652, filed October 1, 1945, the armature is formed on its upper side with a fin which is gripped between pads of rubber so as to damp the armature movement, in the present case this has been found unnecessary owing to the reduced moment of inertia, and the resistance of the record material to the cutter adds suificient damping in most cases.

What is claimed is:

1. A translating device comprising a support, electromagnetic means surrounding said support, a pair of resilient members of non-magnetic material spaced from each other, means for attaching the extremities of said members to said support, an armature of magnetisable material secured to said members by a pair of integral lugs at points displaced from the points of attachment of said members both' longitudinally and laterally thereof, and means for causing said electromagnetic means and portions of said members to vibrate.

2. A translating device as defined in claim 1, wherein said armature vibrates about an axis in its own plane and normal to said members.

MICHAEL LAWRENCE GAYFORD.

REFERENCES CITED The following references are of record in the file of this patent: 

